The nightly builds represent the bleeding-edge of Chrome development . You get access to the latest features as they're introduced into the codebase, but sometimes not all of the bugs have been ironed out, so it's really aimed at developers who want to play around with features that will be making their way into the regular Chrome builds in the future.

You can download Chrome Canary (which updates with the nightly builds) here, and it can be installed alongside the regular version with no problems.